The invention relates to a method of manufacturing ZnS particles by wet-chemical precipitation from aqueous zinc salt solutions whereby ZnS is precipitated on nuclei introduced into the precipitation solution. The invention also relates to ZnS phosphors which can be manufactured by such a method and to a picture tube with a picture screen provided with such a ZnS phosphor.
ZnS particles are used in picture tubes, in particular in colour picture tubes, as phosphors (blue, green) with the relevant dopants. It is essential for the manufacture of high grade phosphor lines or points that the phosphor particles should be applied as densely and as evenly as possible. The particle sizes must not exceed certain limit values. The particle size distribution should be as narrow as possible.
ZnS basic materials for the manufacture of phosphors are usually manufactured by wet-chemical precipitation from Zn salt solutions through the introduction of hydrogen sulphide gas. The problem arises in that case that the continuously dissociated acid of the accompanying anion of the Zn salt continuously lowers the pH value. This leads to yield losses (in relation to the quantity of Zn used) and to unstable conditions in the colloidal chemical system during the reaction if isoelectric points are passed in the course of the reaction. In addition, the precipitation reaction also proceeds very quickly so that the inhomogeneous distribution of the reactants which inevitably arises through the addition of a solution or gas leads to inhomogeneous reaction conditions and thus to an uneven product. Furthermore, such conditions render the control of the nucleation impossible; however, such control is the critical factor if the morphology of the precipitate is to be controlled.
A method of the kind mentioned in the opening paragraph is known from DE-A 1 467 316. In this document, the nuclei are added to a precipitation solution as components of an aqueous suspension.
When nuclei with a narrow particle size distribution are introduced into the precipitation solution, the ZnS particles arising from the nuclei also exhibit a similar narrow particle size distribution, while the particle size can be influenced in a further manufacturing process through a suitable choice of the firing temperature and through the addition of suitable fluxes. The eventual size and size distribution of the ZnS particles is substantially dependent on the size, size distribution, and nature of the nuclei. The surface chemistry of the nuclei also plays a major part.